The accelerometer which is secured to an object whose acceleration is to be detected or measured comprises a pair of magnetic structures each of which has a permanent magnet and a flapper that is flexurally mounted between the magnetic structures, torquer coils being mounted on opposite surfaces of the flapper respectively so as to surround each permanent magnet in each of the magnetic structures, and a case or housing for installing these elements. As the object on which the accelerometer is mounted is accelerated, the flapper tends to remain stationary in space, thus moving relative to the magnetic structures. Two capacitors are formed on each side of the flapper and, as the flapper moves relative to the magnetic structures, the capacitance of one of the capacitors increases and the capacitance of the other capacitor decreases. These capacitors are employed in connection with a balance circuit to produce a DC current which is proportional to the acceleration and is fed back to the coils of the accelerometer to restore the flapper. The DC current proportional to the acceleration is measured.
In order to satisfy severe requirements for operation of the accelerometer and the latest improved technique in this field, the accelerometer must be constructed to minimize effects of temperature changes over wide range in surroundings.
According to changes of temperature in surroundings of the accelerometer, thermal stress will be caused between each of the constitutional elements of the accelerometer which are different from one another in thermal expansion, and the thermal stress will act on the operation of the accelerometer considerably.
In the first place, the magnetic structure of the accelerometer includes a housing and a permanent magnet. The housing may be made of materials which are low in coefficient of thermal expansion such as for example Invar steel, but the permanent magnet cannot be made of material having the same or similar coefficient of thermal expansion as that of the housing. Consequently, the thermal stress produced in the magnetic structure decreases in thermal stability of the accelerometer.
Moreover, thermal stress which acts on the flapper and bobbins to be mounted on the opposite surfaces of the flapper will have a harmful effect on measurement of the accelerometer.
In general, an assembly which is constructed from the magnetic structures and the flapper has been mounted in a cylindrical case with a flange so as to leave an annular clearance or gap between the inner wall of the case and the outer wall of the assembly. The case is inserted into an opening in an instrument board or panel so as to support it by flange of the case.
When the accelerometer mounted on the object in the manner mentioned above is, however, subjected to temperature change in the surroundings, thermal stress will act on the case and the assembly depending upon the difference of coefficient of thermal expansion between the two. Thus, several elements composing the assembly of accelerometer are affected by the thermal stress to cause error in measurement.